I'd imagine you are addressing the player and not the character itself? I mean the character is not the one you are talking to, it's the player behind it. Anyways I don't see a problem when people assume you to be male since probably way over 95% of the player base is male. There is no harm being wrong and you can always correct them. If they keep calling you sir it might be just from the lack of English skills, which seems to be pretty common in this game. If they seem to know English well and keep calling you sir I kinda understand not being cooperative with them.
Also shouldn't you address them with a gender neutral word first because who are you to decide that everyone should be male or female?